With the development of bar code readers and optical character readers (OCR) using near infrared rays, optical character reading has been applied to pressure-sensitive recording sheets, etc. in recent years. In the case of pressure-sensitive recording sheets, heretofore, optical character readers have been used to read characters written or formed on the upper sheet. In general, however, they have not been suitable for or capable of reading characters formed by the rupture of microcapsules.
Recently, various color forming dyes capable of forming dyes having absorptions in the near infrared region have been proposed in JP-B-58-3940 corresponding to U.S. Pat. No. 4,020,056 (the term "JP-B" as used hereinafter means an "examined published Japanese patent application"), JP-A-60-230890 (the term "JP-A" as used hereinafter means an "unexamined published Japanese patent application") and JP-A-59-199757. When these dyes are used, the characters on several of the pressure-sensitive recording sheets (up to 5 to 6 sheets) can be read by the optical character reader, but there are problems in that these dyes are colored yellow, the background tends to fog, and the dyes' developability is poor. Accordingly, these dyes are unsuitable for practical use.
Recently, color forming divinyl compounds which absorb in the near infrared region and are free from the above-described problems have been proposed in JP-A-62-243653 corresponding to EP-A-242170.
When these divinyl compounds are microencapsulated and used as color formers, an improved pressure-sensitive recording sheet can be obtained on which the characters on all the paper sheets usually can be read by the optical character reader using near infrared rays. In some instances, however, it is difficult to read certain recorded characters depending on their forms, etc. Particularly, when the number of sheets is increased, the likelihood increases that the optical character reader cannot read the characters recorded on the forth, fifth, and subsequent sheets, fairly.